1. Field of the Disclosure
Embodiments of the present disclosure may include a surface emitting laser device having a double-channel structure, and more particularly, a vertical external cavity surface emitting laser (VECSEL) that may obtain a uniform current density in an active layer using two current injection channels.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating a structure of a vertical cavity surface emitting laser (VCSEL) that emits a laser beam in a direction vertical to a substrate. Referring to FIG. 1, a conventional VCSEL 100 has a structure in which a lower distributed Bragg reflector (DBR) layer 113, an active layer 114, and an upper DBR layer 116 are consecutively stacked on a substrate 111 made of n-GaAs. The lower and upper DBR layers 113 and 116 are reflection layers having high reflectivity with respect to the oscillation wavelength of the laser. The lower DBR layer 113 is an n-type DBR layer doped with an n-type dopant, and the upper DBR layer 116 is a p-type DBR layer doped with a p-type dopant. A metal contact 117 for applying current to the active layer 114 is formed in the upper DBR layer 116. In this structure, when current is applied to the active layer 114, light is emitted from the active layer 114 due to the recombination of holes and electrons in the active layer 114. The light is emitted through the upper DBR layer 116 after being repeatedly reflected between the upper DBR layer 116 and the lower DBR layer 113 and amplified in the active layer 114.
However, in the case of the VCSEL 100, since a resistance in a horizontal direction is considerably greater than a resistance in a vertical direction, the current density is not uniform over the active layer 114 and a current crowding phenomenon, i.e., the concentration of current near the edge of an aperture, which is a light emitting region of the active layer 114, occurs. A curve indicated by “A” in FIG. 1 is a current density profile of the active layer 114. As is seen from the curve “A”, the current density is remarkably reduced in a central region of the active layer 114 due to the high resistance in the horizontal direction. This phenomenon causes a single transverse mode oscillation to occur in the VCSEL 100. To solve this problem, as depicted in FIG. 1, conventionally the size of the aperture of the active layer 114 is limited to approximately 5 μm by forming an oxide layer 115 by selectively oxidizing the periphery of a lower surface of the upper DBR layer 116. An ion implantation layer can be used instead of the oxide layer 115. Therefore, the conventional VCSEL 100 has a drawback in that its output power is merely a few mW due to the small aperture.
A vertical external cavity surface emitting laser (VECSEL) is a laser device designed to realize a high output operation. The VECSEL can obtain a power of at least a few hundreds mW due to a gain region enlarged using an external mirror.
FIG. 2 is a cross-sectional view illustrating a VECSEL. Referring to FIG. 2, a conventional VECSEL 120 includes a substrate 121, a lower DBR layer 122, an active layer 123, an upper DBR layer 124, and a concave external mirror 126. Here, laser cavities are respectively formed between the lower DBR layer 122 and the upper DBR layer 124 and between the lower DBR layer 122 and the concave external mirror 126. In this structure, light generated in the active layer 123 reciprocates in the active layer 123 while being repeatedly reflected between the lower DBR layer 122 and the upper DBR layer 124 and between the lower DBR layer 122 and the external mirror 126. A portion of the light having a wavelength λ2 amplified in the active layer 123 through the above process is emitted as a laser beam to the outside through the external mirror 126, and the other portion of the light is reused for optical pumping via reflection.
There are two methods of exciting the active layer 123 in the VECSEL 120. A first method is, as depicted in FIG. 2, an optical pumping method in which a light beam λ1 having a shorter wavelength than the laser beam λ2 emitted from the active layer 123 is directed to enter the active layer 123 using a pump laser 127. The other method is, like the VCSEL depicted in FIG. 1, an electric pumping method in which current is applied to the active layer 123 through a metal contact formed on the upper DBR layer 124. However, when the electric pumping method is used, the drawbacks of the VCSEL described above still occur. Furthermore, in the case of the VECSEL 120, the aperture is as large as 20-100 μm for a high output power. Therefore, the problem that current is concentrated near the edge of the aperture becomes more severe, and it is more difficult to achieve single transverse mode oscillation for a higher output power.
A laser device in FIG. 3 developed to overcome the above-described drawbacks is disclosed in U.S. Pat. No. 6,243,407 to Aram Mooradian, entitled “High power laser devices” and filed on Jul. 7, 1997. Referring to FIG. 3, a laser device 130 includes a p-type DBR layer 131, an active layer 132, an n-type DBR layer 133, a substrate 134, and an external mirror 138. A circular contact layer 135 is formed on a lower surface of the p-type DBR layer 131, and an annular contact layer 136 is formed on an upper surface of the substrate 134. Current is applied to the active layer 132 through the contact layers 135 and 136. The substrate 134 is formed of n-GaAs, which is transparent to the oscillation wavelength of the laser device, to a thickness of approximately 500 μm. In this structure, laser cavities are respectively formed between the p-type DBR layer 131 and the n-type DBR layer 133 and between the p-type DBR layer 131 and the external mirror 138. A second harmonic generating (SHG) crystal 137, which doubles a frequency of light, can be additionally disposed between the external mirror 138 and the substrate 134.
As seen from FIG. 3, the laser device 130 disclosed in the above U.S. Patent publication is designed such that light generated in the active layer 132 passes through the substrate 134. That is, the substrate 134 is disposed in the laser cavity formed between the p-type DBR layer 131 and the external mirror 138. This structure mitigates the concentration of current described above by allowing the current to diffuse in a horizontal direction while flowing through the substrate 134, which is relatively thick.
However, the laser device 130 depicted in FIG. 3 has the following drawbacks. That is, the absorption of free carriers by n-GaAs, which is commonly used as a material for the substrate 134, limits the output power and efficiency of the device. This loss is not negligible because the thickness of the n-GaAs substrate 134 through which the beam passes reaches a few hundreds of μm.
Also, in the case of a conventional VECSEL, about 30% of the total optical energy being amplified in the active layer 132 distributes in the laser cavity between the upper and lower DBR layers, and about 70% of the optical energy distributes in the laser cavity between the lower DBR layer and the external mirror. Meanwhile, in the case of the laser device depicted in FIG. 3, about 30% of the optical energy is distributed in the laser cavity formed between the DBR layer 131 and the external mirror 138 to reduce a loss due to the absorption of free carriers. However, the conversion efficiency of the SHG crystal 137 disposed between the substrate 134 and the external mirror 138 increases in proportion to the intensity of the optical energy. Accordingly, the overall efficiency of the laser device 130 depicted in FIG. 3 decreases. The conversion efficiency of the SHG is further reduced since the distance between the active layer 132 and the external mirror 138 is great and light reaching the SHG crystal 137 is dispersed to some extent.
Also, it is very difficult to meet resonant conditions since the substrate 134 and air are present between the external mirror 138 and the p-type DBR layer 131. Also, as the optical path is longer, a greater degree of precision is required when forming the concave surface of the external mirror 138 so that light reflected by the external mirror 138 can converge on the p-type DBR layer 131.
Furthermore, a process of manufacturing the laser device is complicated since the manufacturing is performed on both upper and lower surfaces of the substrate 134, not only one surface of the substrate 134.